http://revach.net/article.php?id=3443

Section: Parshas HaShavua   Category: Quick Vort
Parshas Mishpatim: Rav Yehonoson Eibushitz - The Tzira As A Secret Weapon?
"V'Shalachti Es HaTzira Lifanecha V'Geirsha Es HaChivi V'Es HaKenana...", I will send hornets ahead of you and they will drive out the Chivi, the Kena'ani.... (Mishpatim 23:28).  These hornets says the Medrash blinded their eyes above and sterilized them below.  Why did Hashem specifically use this as weapon to drive out Kena'an?

Rav Yehonoson Eibushitz explains that this was Mida K'Neged Mida.  Kena'an was the one who sterilized his grandfather Noach, so that he would not have more children.  He also made himself blind to Noach, who was lying immodestly on the floor, and he did not cover him.  Therefore the Tzira was fitting retribution for this aveira.